1. Field of the Invention
The present invention relates to a method of and an apparatus for manufacturing an optical information recording medium.
2. Description of the Prior Art
Technology in which playback and recording of high-density information are performed by using a laser beam is known and is put to practical use mainly as an optical disk. Optical disks can be roughly classified into read-only type, write-once read multiple type and rewritable type. The read-only optical disk is commercially available as a compact disk for recording musical information and a laser disk for recording information on images, while the write-once read multiple type optical disk is commercially available for storing document files and still picture files. Furthermore, the rewritable type optical disk is commercially available for storing data files for a personal computer.
The optical disk usually has an arrangement in which an information layer is provided on a principal face of a transparent resinous substrate of 1.2 mm in thickness and a protective film, such as an overcoat, is provided on the information layer or alternatively, an arrangement in which a protective sheet having the same shape as a substrate is bonded to the substrate.
Meanwhile, in recent years, use of a shorter laser wavelength and an objective lens having a larger numerical aperture (NA) has been studied in order to achieve higher density of the optical disk. However, the shorter wavelength and the larger numerical aperture reduce an allowable value of an angle of inclination, (tilt) of the optical disk relative to a direction of incidence of the laser beam. Reduction of thickness of the substrate is effective for increasing the allowable value of the tilt. For example, in a digital video disk (DVD) having a laser wavelength of 650 nm and a numerical aperture of 0.60, the substrate has a thickness of 0.6 mm. Since mechanical strength of the single substrate of 0.6 mm in thickness is small, the two substrates are bonded to each other such that information recording faces of the substrates confront each other.
In order to bond the two substrates to each other, a method is mainly employed in which radiation cure resin is coated on one substrate, the other substrate is brought into close contact with the one substrate and then, radiation is irradiated over the substrates so as to cure the radiation cure resin. This method is referred to as a “radiation cure method”, hereinafter. Meanwhile, ultraviolet (UV) rays are generally used as radiation. Generally in the radiation cure method, radiation cure resin is coated on one substrate annularly by rotating the one substrate at low speed and the other substrate is placed on the one substrate such that the two substrates are formed integrally. Subsequently, after the radiation cure resin has been fully diffused between the two substrates by rotating the two substrates at high speed, radiation is irradiated to the substrates so as to cure the radiation cure resin.
However, in this method, since a position of diffusion of the radiation cure resin to an inner periphery of the substrates changes based on a position of coating of the radiation cure resin, a timing of placing the substrates on each other, high-speed rotational conditions of the substrates, etc., it is difficult to stop the resin at a predetermined radial position of the substrates. In a case where the resin is excessively diffused to the inner periphery of the substrates, the resin protrudes into central bores of the substrates. If the resin is cured in this state, the substrates become eccentric relative to a turntable when mounted on the turntable. Therefore, in this case, the resin should be cured after having been wiped from the central bores of the substrates. Meanwhile, in a case where the resin is insufficiently diffused to the inner periphery of the substrates, a disk clamp area of the substrates, which is used for clamping the substrates to the turntable, is not filled with the resin and thus, the mechanical strength of the optical disk becomes small.
In order to solve this problem, Japanese Patent Laid-Open Publication No. 8-321074 (1996) proposes that a stopper for preventing the radiation cure resin from protruding into the central bores is provided at an innermost periphery of the substrate. For example, an annular recess is formed at an inner peripheral side of the disk clamp area on at least one of opposed faces of the substrates. Thus, when the resin has been diffused to the inner periphery of the substrates, the resin is received in the recess acting as a resin reservoir and therefore, is prevented from being diffused further to the inner periphery of the substrates. Namely, even if the resin is filled in the disk clamp area, the resin does not protrude into the central bores of the substrates. Accordingly, it is possible to stably manufacture an optical disk having a large mechanical strength.
In order to manufacture an optical disk, a method is generally known in which two substrates are bonded to each other with ultraviolet (UV) cure resin. This method has such features that (1) since air bubbles or the like are not contained in the resin, an external appearance of the optical disk is good and (2) since the resin is instantaneously cured upon irradiation of UV rays thereto, working efficiency is excellent and tact time can be shortened.
Further, thin substrates represented by the DVD have been used in recent years. Since the mechanical strength of the thin substrate is small, it is desirable that the thin substrates are bonded to each other by filling the resin also in the disk clamp area of the substrates so as to be formed integrally. To this end, the UV cure resin should be coated on a neighborhood of the central bores of the substrates.
However, it has been difficult to stop the resin at a predetermined radial position of the substrate at all times. This is because diffusion speed of the UV cure resin to an inner periphery of the substrates changes based on a position of coating of the UV cure resin, timing of bonding of the substrates, rotational conditions of the substrates, change of viscosity of the UV cure resin due to temperature changes, etc. In a case where the resin is excessively diffused to an inner periphery of the substrates, the resin protrudes into the central bores of the substrates. If the resin is cured in this state, the substrates become eccentric relative to a turntable when mounted on the turntable. Therefore, in this case, the resin should be cured after having been wiped from the central bores of the substrates. Meanwhile, in a case where the resin is insufficiently diffused to the inner periphery of the substrates, the resin is not filled in the disk clamp area of the substrates and thus, mechanical strength of the optical disk becomes small as described above.
Meanwhile, Japanese Patent Laid-Open Publication No. 8-321074 (1996) discloses a method in which the radiation cure resin is filled in the disk clamp area stably without projecting into the central bores of the substrates. However, in this prior art document, it is essential that the stopper is provided on the substrate. In order to form the stopper on the substrate, a method in which an annular protuberance is provided on the substrate by printing or a method in which an annular recess is provided on the substrate by cutting the substrate is disclosed. At any rate, it is necessary to provide a step of working the substrate. However, the provision of the step of working the substrate results in adherence of dust thereto and rise of production cost.
Meanwhile, it is also possible to form the stopper on the substrate preliminarily. However, in a case where the substrate is manufactured by, for example, transfer from a stamper, optical characteristics of the substrate and properties of transfer from the stamper to the substrate may be adversely affected by the stopper. Namely, if an annular protuberance is provided on the stamper or a stamper holder, an annular recess acting as the stopper is formed on the substrate. However, in a case of an optical information recording medium, especially an optical disk, the substrate is manufactured by injection molding. Thus, if the protuberance is preliminarily provided on the stamper, flow of the resin at the time of molding of the substrate is different from that in a case where the protuberance is not provided on the stamper, so that double refraction and formation of signal recording pits and signal recording grooves of the manufactured substrate may be insufficient. Furthermore, in a case where a position of the stopper is required to be changed, the position of the protuberance on the stamper or the stamper holder should also be changed. As a result, it has been practically difficult to control resinous filling to an arbitrary position.
Meanwhile, in the case where the two substrates are bonded to each other such that the information recording faces of the substrates confront each other as described above, an optical disk in which a reflective layer made of aluminum or the like, as in prior art, is formed on the information recording face of one of the substrates and a thin translucent reflective layer made of gold or the like, is formed on the information recording layer of the other of the substrates such that playback on the two information recording faces is performed from the substrate having the translucent reflective layer is proposed and put to practical use. Another optical disk formed by the two substrates is also proposed in which not the metallic reflective layer, but a thin rewritable recording layer is formed on the information recording layer.
Furthermore, in order to achieve higher density, the use of a bluish purple laser beam source having a wavelength of about 400 nm is also proposed. In this case, an arrangement of the two substrates can be obtained in the same manner as described above.
In the DVD, the substrate has a thickness of 0.6 mm. However, a method is also proposed in which the substrate is set at a thickness of about 0.1 mm and a quite minute laser beam spot is formed by using a lens having an NA of about 0.85, so as to perform recording and playback of signals. Since it is difficult to provide a signal recording layer on the substrate of 0.1 mm in thickness, a thin translucent sheet is generally bonded, by using UV cure resin, to a substrate of about 1.1 mm in thickness, which is provided with the signal recording layer, such that recording or playback of signals is performed from the sheet. At this time, a sum of a thickness of the sheet and that of the UV cure resin is designed to reach 0.1 mm.
In order to bond the substrates to each other, there is a method in which UV cure resin is coated on one substrate, the other substrate is brought into close contact with the one substrate and then, UV rays are irradiated on the substrate so as to cure the UV cure resin. Generally in this method, the UV cure resin is annularly coated on the one substrate by rotating the one substrate at low speed and the other substrate is placed on the one substrate such that the two substrates are formed integrally. Subsequently, after the UV cure resin has been fully diffused and spread between the two substrates by rotating the substrates at high speed, UV rays are irradiated to the substrates so as to cure the UV cure resin.
However, in this conventional method, thickness of the UV cure resin varies based on a position of coating of the UV cure resin, timing of placing the substrates on each other, high-speed rotational conditions of the substrates, etc. These variations take place not only between optical disks, but in one optical disk. Generally, since the UV cure resin is spread by centrifugal force produced by high-speed rotation of the substrates, the thickness of the UV cure resin is small at an inner periphery of the substrates but is large at an outer periphery of the substrates. Such distribution of thickness of the UV cure resin poses a problem in a case where recording or playback is performed through a layer of the UV cure resin. Namely, in the case where recording or playback is performed from one of the two substrates bonded to each other or the thin translucent sheet is bonded, by using the UV cure resin, to the substrate having the signal recording layer such that recording and playback of signals are performed from the sheet. Namely, since variations of thickness of the UV cure resin result in variations of an optical path length of a laser beam, a shape of a laser beam spot on the signal recording layer varies, thereby resulting in variations of recording and playback characteristics. This adverse effect becomes greater in a case where the laser beam spot is reduced in diameter. Namely, in a case where a bluish purple laser beam is employed as the laser beam or an objective lens having an NA as large as 0.85 is used.